Still miss my 99, which made a great track day toy. I find the NC more comfortable for daily driving but certainly any generation of Miata can get the dual purpose job done of track days and daily driving. But if your Bro can't stand the idea of you both having NC, seems to me it is his decision to get an NB. Not clear to me why you would sell your NC because of his odd thing about both of you having the same car.
My thinking is exactly opposite of his, I think it really cool if you both have the same car, both in NC. You could trade bits back and forth, swap wheel and tire selections, sways, compare and contrast alignment settings, etc. If you both have an NC, just think of all the fun you could have testing bits and comparing setups on the same platform, racing each other in the same SCCA autocross class, etc. Perhaps you challenge him to join you next season in STR class and you guys have a blast with some friendly brother to brother competition.